This invention relates to automatic gain control apparatus and, in particular, to an automatic gain control system wherein signal gain is a function of the distance between communicating stations.
It is desirable, in many applications, to communicate electrical information signals between remote locations over a communication channel having minimal equipment thereon. Such applications include instrumentation telemetry wherein data is transmitted from remote sensing stations to a central control station and control information may, additionally, be returned to various controlling locations. Other applications include pipe line communication links, remote telemetry links and other instrumentation and supervisory/control links.
Each of the aforementioned communication links may be characterized as having no or minimal signal amplification devices included in the communication channel. This lack of equipment in the communication channel is preferred to minimize the cost of constructing and maintaining such channels. This is especially so for those applications wherein the communication channels are used primarily for communication between maintenance personnel or equipment wherein a high quality information link is not of paramount importance.
In systems wherein the distance between communicating stations is variable and, for the most part, unpredictable, signal attenuation presents an acute problem. In particular, audio signals are subject to substantial attenuation whereby the intelligence information conveyed thereby is severely distorted. Such signal attenuation is primarily attributed to the total resistive impedance included in the signal channel between the communicating stations. Since the basic communication channel consists of a loop formed by, for example, two electrical conductors, the total resistive impedance is equal to the inherent loop resistance of the conductors.
This problem of signal attenuation can be obviated by using a plurality of signal amplifiers in the communication channel to successively amplify the communicated signal so as to maintain a desirable signal gain. However, this technique significantly adds to the cost of constructing and maintaining the signal channel which was initially intended to be of minimal cost. Also, by providing such amplifying devices, such as repeater stations, at preselected locations along the communication channel, the desired flexibility in using that channel is unduly constrained. That is, for applications wherein signaling is to be attained between a given location, such as a central station, and a remote location, it is preferable to permit the remote location to be disposed at any arbitrary position along the conductors. However, by providing amplification devices at preselected locations along the conductors, the position of the remote signaling station must be fixed to assure a matching of the system transmission characteristics. It can be appreciated that if the remote station must be fixed to predetermined locations, the use of the communication channel in maintenance operations, for example, is not readily facilitated. Furthermore, suitable power supplies must be provided for these amplification devices, thus adding to the cost and complexity of the signal channel.
Alternatively, the problem of signal attenuation might be attended to by the use of a fixed gain amplifier at one of the communicating stations. However, since the resistive impedance between the stations is a function of the distance, the fixed gain amplifier might be inadequate to suitably amplify the signals that emanate from a distant location whereas signals transmitted from a nearby location might be "over-amplified" and distorted. If attenuator pads are disposed at the remote locations to account for the different impedances, the fixed gain amplifier would have to operate at maximum gain in all instances, with the result that the signal-to-noise ratio would be relatively poor. It is believed that if the fixed gain amplifier is replaced by an amplifier having a manually adjustable gain, less than perfect results would be attained by an operator who must adjust the amplification factor to compensate for variable distances.
Until recently, it had been very difficult to control the gain of a semiconductor amplifier to permit the proper amplification of signals transmitted over widely variable distances via electrical conductors due to the wide input dynamic range required. Primarily, it had been most difficult to change the gain of such an amplifier by changing either the voltage or current parameters thereof. However, with the advent of the transconductance amplifier, a device has now been provided that exhibits a very linear change of transconductance proportional to an input current. The operational characteristics of such transconductance amplifiers are exploited by the present invention to thereby provide a variable gain amplifier having a gain determined by the distance between communicating locations and having a large input signal dynamic range capability.